class cls extends Array {
    sum() {
        return this.reduce(function reduce(acc,curr) {
            return acc+curr
        },0)
    }
}
const x =new cls(3)
const y = new Array(3)
const z = cls.of(3)
console.log(x.length,y.length,z.length)
const a = 'abc'
const b = new String('abc')
console.log(a===b,a.substr(),b.substr())
console.log('some string'.includes("") === false)
console.log('some string'.replace('s', '') === 'ome string')
console.log('sOme string'.replace(/o/i, '') === 'sme string')
